However, in jQuery 1.3 beta 1, storing an empty string ("") makes jQuery.data() return null. Furthermore, if the value was not defined, it also returns null instead of undefined, so it is not possible to know if a null value was actually stored or if no value was stored at all, which makes it practically impossible to store null as value as well.

This means that code which stores empty string values will break because it will get a null back instead of a string, and code which relied on the return value being different than undefined to confirm that it was set will also break.